AB or ABC block copolymers having at least one block comprising olefinic double bonds (polybutadiene, polyisoprene, and the like) can be used alone or as a blend with other polymers, such as PVDF, PVC, PVCC, and the like, for improving some of their properties. However, the presence of the block comprising olefinic double bonds renders them sensitive to light, to some oxidizing agents and to heat. The selective hydrogenation of this block makes it possible to prepare novel materials comprising polyolefins while improving their stability (towards light, towards oxidizing agents and towards heat) and their mechanical properties. This hydrogenation also results in a modification in the physical properties of the polymer by creating a block comprising fewer olefinic double bonds, which block can become a semicrystalline block. Furthermore, the presence of a polyolefin chain renders them compatible with a broader range of polymers (including polyolefins), which represents a very large potential market.
The hydrogenation of these block copolymers can be carried out by non-catalytic methods, generally performed in the presence of hydrazine derivatives, such as, for example, p-toluenesulphonylhydrazine. Although these methods do not require a reactant which operates under pressure, their industrial implementation cannot be envisaged because of the high cost of the p-toluenesulphonylhydrazine reactant.
Block copolymers can also be hydrogenated by heterogeneous catalysis. However, as heterogeneous catalysts have a low activity, it is necessary to operate at a high temperature and a high hydrogen pressure and to use large amounts of catalyst. These operating conditions can result in a decomposition or in a crosslinking of the polymer and in a decrease in the selectivity of the hydrogenation (hydrogenation of functional groups other than the olefinic double bonds: esters, aromatic double bonds, and the like).
The hydrogenation can also be carried out in a homogeneous medium under milder conditions by using, as catalysts, noble metal complexes (Wilkinson's catalyst, and the like) or cobalt or nickel salts with reducing agents (triethylaluminium, butyllithium, and the like). The use of a very small amount of catalyst can result in an economical process, even if the catalyst is not recovered; however, the latter partly remains in the polymer, which can be harmful to its properties and thus require its purification. On the other hand, when it is necessary to use a large amount of catalyst, it has to be recovered for recycling.
The hydrogenation of block copolymers having a polybutadiene block by homogeneous catalysis in the presence of Wilkinson's catalyst has formed the subject of many publications, in particular Patent Application DE 4 240 445, the thesis by C. Auschra at the University of Mainz (1992), entitled “Synthese von neuartigen Multi block copolymer en und deren Verwendung in Polymerlegierungen” [Synthesis of novel multiblock copolymers and their use in polymer alloys], the articles by C. Auschra et al. in Polymer Bulletin, 30 (1993), 257–264 and 305–311 and in Macromolecules, 26 (1993), 2171–2174, and an article by R. Stadler et al. in Macromolecules, 28 (1995), 3080–3097. The copolymers used have a polybutadiene block formed of butadiene predominantly possessing a 1,2-microstructure (approximately 90%), which hydrogenates much more easily than polybutadiene predominantly possessing a 1,4-microstructure (85 to 89%). Furthermore, the amount of Wilkinson's catalyst used is high (8 000 molar ppm per mole of double bond).
The hydrogenation of a polystyrene-polybutadiene-poly(ε-caprolactone) triblock copolymer in the presence of Wilkinson's catalyst (10 000 ppm), the polybutadiene block predominantly possessing a 1,4-microstructure, is disclosed in Patent Application DE 19643889 and in Macromol. Chem. Phys., 199 (1998), 1063–1070.
The selective hydrogenation of the polybutadiene block of an NBR (Nitrile Butadiene Rubber) copolymer without affecting the nitrile groups is described by L. A. Müller et al. in Macromol. Rapid Commun., 19 (1998), 409–411. The reaction, catalysed by the RuHCl(CO)(PCy3)2 complex, is carried out in a two-phase medium (ionic liquid+organic solvent); the ionic liquid is 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4) and the NBR is dissolved in toluene. The ionic liquid solution comprising the catalyst can be recycled several times.
This process, which makes it possible to easily recover the catalyst and to recycle it, also forms the subject-matter of Patent Application BR 98 02101, Example 2 of which illustrates the same hydrogenation of an NBR with the same ionic liquid (bmimBF4) but the general features of which include a good number of other catalysts, of other unsaturated copolymers and of other ionic liquids, such as those in which the cation is a quaternary ammonium or phosphonium group and the anion derives from a Lewis acid, such as, for example, the AlCl4−, RSO3−, BF4−, ZnCl42−, ZnBr42−, PF6−, CuCl2− or FeCl3− anions, and the like.
As indicated above, the hydrogenation of copolymers comprising a butadiene block predominantly possessing a 1,4-microstructure is difficult. Thus, when the method of Example 2 of document BR 98 02101 is applied to the hydrogenation of such a copolymer (SBM triblock) with Wilkinson's catalyst at a moderate temperature (60° C.), a degree of hydrogenation of the order of 30% only is obtained.